Quickest start to extending Java Object - Doom etc

Newbie & Debugging Questions
#1

I have a passing knowledge only of Java. Have one working game in Java, and would like to build it onto a recognisable open source Java version of eg Doom.

Able to add multiple shooters operated by a single player;

Strong preference for availability of less violent version.
Srong preference for stable version.
Thanks for any leads,

…Contender. ;D

#2

Whats your question?

Dr. A>

#3

Fair question.

Question is, can anyone suggest an existing project(s) from which I can start and which meet the above criteria.

Thanks, …Contender

#4

I know of http://javaisdoomed.sourceforge.net/, might provide you with a good starting point.